In a hybrid drive vehicle in which an internal combustion engine is connected to an electric motor via a clutch, the electric motor cranks the internal combustion engine by connecting the clutch.
In an initial state of a factory shipment stage of a hybrid drive vehicle, an intake air amount of an internal combustion engine may not reach an amount equivalent to a target idle rotation speed due to product variations or high friction of the internal combustion engine immediately after manufacturing when the electric motor first starts the internal combustion engine.
If the intake air amount is below an amount necessary to meet the target idle rotation speed, there is a possibility that the internal combustion engine stalls without being able to maintain idle rotation when the clutch is disengaged after cranking and a rotational torque applied to the internal combustion engine by the electric motor disappears.
If the intake air amount is set to be slightly large in advance, an engine stall associated with the disengagement of the clutch can be prevented. In this case, however, a possibility of a rapid increase in an engine rotation speed after the disengagement of the clutch becomes higher.
Concerning the start of an internal combustion engine in a hybrid drive system, JP H09-144586A, published in 1997 by the Japan Patent Office, proposes a method for stabilizing a rotation speed of an internal combustion engine immediately after a start within a short time period. This method detects whether or not a feedback correction amount of an intake air amount of the internal combustion engine is limited by a limit value and changes the limit value or a learnt value if the feedback correction amount is limited by the limit value.